p-xylene is one of the major basic organic feedstocks in petrochemical industry and has widespread applications in many fields such as chemical fiber, synthetic resin, pesticide, medicine, plastic, etc. The typical process for producing p-xylene (pX) is to separate p-xylene in the xylene stream containing ethylbenzene, i.e. C8 aromatics in thermodynamic equilibrium produced in the catalytic reforming of naphtha by multi-stage cryogenic crystallization separation or molecular sieve simulation moving bed separation (abbreviated as adsorptive separation) from its isomer mixture with near boiling points. A C8A isomerization (abbreviated as isomerization) process is generally used to isomerize o-xylene and m-xylene to p-xylene. The use of disproportionation of toluene, or the disproportionation and alkyl transfer reaction of toluene and C9+ aromatics (C9+A) to produce benzene and C8A, and thereby increase the output of C8A is an effective route for increasing the output of p-xylene.
So far, the rather typical and mature processes relating to toluene disproportionation in the world include the Tatoray traditional toluene disproportionation process industrialized in the end of 1960s, the MTDP process developed in the end of 1980s, and the S-TDT and TransPlus processes developed in recent years. The selective disproportionation of toluene is a new route for producing p-xylene. Since the selective disproportionation of toluene on modified ZSM-5 catalysts can produce benzene and C8A with a high concentration of p-xylene, only one simple step of cryogenic separation is enough for separating most of the highly pure p-xylene. In recent years, along with the improvement of the catalyst performance, this process has made a great progress. The typical processes include the MSTDP selective disproportionation process of toluene industrialized late 1980s and the pX-Plus process developed in recent years.
In the industrialized toluene selective disproportionation process-MSTDP, a treated ZSM-5 mesoporous molecular sieve is used as the catalyst to treat a toluene feedstock yielding C8A with a high concentration of p-xylenes (85-90% by weight, the same bellow except otherwise noted) and nitration grade benzene. In the pX-Plus process, the report on the industrialization of which has not seen, the major indices are a pX selectivity of 90% and a benzene/pX mole ratio of 1.37 in case of a toluene conversion of 30%.
However, in this kind of processes for toluene selective disproportionation, a high p-xylene selectivity is accompanied by a harsh requirement for the feedstock selection, and only toluene can be used as the feedstock, while C9+A has no use for this kind of processes, and at least it cannot be directly used. Besides, this process also produces a great amount of benzene as a by-product, resulting in a low yield of p-xylene. This is a vital shortcoming of the process for selective disproportionation.
The feed to the reactor of the typical Tatoray process is toluene and C9 aromatics (C9A), and the content of C10+ hydrocarbons (C10 and higher hydrocarbons) must be strictly controlled. To increase the economic benefit of the device and decrease the energy and material consumption, further study and optimization of the Tatoray process have been carried out with the focus placed on the kernel technique—the preparation of the catalyst, the improvement of the whole performance of the catalyst such as the increase of the weight space velocity, the elongation of the operation cycle of the catalyst, and the increase of the average molecular weight of the aromatics feedstock. The increase of the average molecular weight benefits the increase of C8A, but in order to maintain a certain conversion, i.e. a certain catalyst activity, too high a content of heavy aromatics would certainly lead to an enhancement of the side-reactions especially the hydrodealkylation reaction, thereby lead to the increase of the benzene product in the reaction product, the decrease of the C8A/Ben ratio, more loss of aromatics, and therefore lead to less C8A and more Ben when the same feedstock is treated. The reason why the toluene disproportionation unit is necessary for an aromatics integrated device is that it has a function to provide C8A. The increase of Ben and the decrease of C8A are obviously unfavorable to the whole aromatics integrated device. These shortcomings have already restricted the development of this kind of processes.
The literature based on the Tatoray Process includes U.S. Pat. No. 4,341,914, CN98110859.8, U.S. Pat. No. 2,795,629, U.S. Pat. No. 3,551,510, CN97106719.8, etc. FIG. 1 is the process flow of U.S. Pat. No. 4,341,914, wherein 1 is the xylene tower I, 2 is the heavy aromatics tower, 3 is the reaction zone, 4 is the benzene tower, 5 is the toluene tower, 6 is the xylene tower II, 7 is C9A, 8 is the C8+A feedstock, 9 and 10 are toluene, 11 is benzene, 12 and 13 are C8A, 17 and 19 are the streams rich in C10+ hydrocarbons, and 18 is the stream rich in C9A. In this process, although a part of C10A in the reaction product is returned to the reaction zone along with the recycled C9A (stream 18) to partly make use of the produced C10A by the reaction itself to suppress the production of a greater amount of C10+ hydrocarbons in the reaction, the C10+ hydrocarbons in the C8+A feedstock can not be utilized and a part of C9A in the C8+A feedstock is withdrawn from the bottom of the heavy aromatics tower along with C10+ hydrocarbons (stream 19). Due to the limit of the catalyst performance, this process also has a harsh requirement for the selection of the feedstock, i.e. the effluent from the top of the heavy aromatics tower (tower 2)—the C9A stream (stream 7) must contain less than 1% of indan (IND), thereby resulting in the aforesaid loss of C9A and only part utilization of C10A produced by the reaction itself, while the C10+ hydrocarbons in the C8+A feedstock can not be utilized.
FIG. 2 is the process flow of CN98110859.8, wherein 1 is the xylene tower I, 2 is the heavy aromatics tower, 3 is the reaction zone, 4 is the benzene tower, 5 is the toluene tower, 6 is the xylene tower II, 7 is the o-xylene tower, 8 is the C8+A feedstock, 9 is the fresh toluene, 12 and 13 are C8A, 14 is the stream rich in C9A, 15 is C11 and higher hydrocarbons (C11+ hydrocarbons), 16 is the recycled toluene, 17 is benzene, 19 is o-xylene, 20 is the C9A+ containing or not containing o-xylene. Although this process has overcome many shortcomings of the above patent and has the advantages of permission of high contents of indan and C10+ hydrocarbons in the feedstock, etc, the amount of the by-product—benzene is still large.
It is readily seen from summarizing the above processes that all these patents are formed by reasonable modifications of a specific catalyst for toluene disproportionation and alkyl transfer in one or more aspects such as the ability for alkyl transfer of heavy aromatics or the separation scheme of the reaction product, but these modifications have not broken though the limit of the original idea of the Tatoray process. The common shortcomings are that benzene is inevitably produced as a by-product when using toluene or toluene and C9+A to produce C8A and increase the yield of p-xylene and that the heavy aromatics can not be effectively utilized.